Primary osteoarthritis (OA) has a very strong genetic component; however, only a small number of OA risk alleles have been identified. In this proposal we focus on telomere shortening as an epigenetic factor contributing to primary OA. The silencing effect of telomeres on the genes located nearby is known as telomere position effect and is disrupted by telomere shortening, which was detected previously in patients with OA. The objective of this application is to prove or refute the concept that telomere shortening has a mechanistic relationship to the pathogenesis of OA and to show a therapeutic effect of SIRT6 activators. Our central hypothesis is that telomere shortening contributes to the development of OA through upregulated expression of telomere-proximal genes, which is antagonized by SIRT6. We also propose that telomere shortening in cartilage and synovium triggers telomere-associated secretory phenotype (TASP) in the joint and expression of telomeric repeat-containing RNA (TERRA). This hypothesis has been formulated on the basis of the current literature and preliminary data revealing increased SIRT6 expression in OA cartilage with short telomeres, showing significant shortening of telomeres in affected versus unaffected cartilage obtained from 50 patients with knee OA during joint replacement surgery, indicating enrichment in differentially expressed genes located close to telomeres in cartilage with very short telomeres and showing a suppressive effect of NMN on chondrocyte senescence. Guided by preliminary data, this hypothesis will be tested by pursuing three specific aims: 1) Determine the effect of SIRT6 activators such as nicotinamide mononucleotide (NMN) on progression of OA in affected chondrocytes; 2) Determine if telomere extension can silence upregulated telomere-proximal genes; and 3) Determine if TERRA transcripts are present in synovial fluid exosomes of OA patients and if their level has a direct effect on the inflammatory response of chondrocytes. The proposed research is significant because it indicates telomere position effect affected by telomere shortening as the new important mechanism contributing to cartilage aging and OA pathology. The novelty of this project is that it explores a new epigenetic mechanism contributing to etiology of OA.